Field of the Invention
The present invention relates to an inverter circuit which converts DC power of a solar battery, a fuel battery, or the like which is a distributed power source or power supply, to AC power, with high efficiency, so as to be interconnected to a commercial power system or power grid.
Description of Related Art
Conventionally, it is known that DC power generated by a distributed power source such as a solar battery is converted to AC power matched to a commercial power system by an inverter circuit, and the converted AC power is supplied to the power system. The inverter circuit without a transformer therein includes: a smoothing capacitor on the input side, connected in parallel to the solar battery; a bridge circuit which has four switching elements and four flywheel diodes or free wheeling diodes respectively connected in reversely parallel to the switching elements and which converts DC power to AC power through switching; an interconnection reactor and a capacitor on the output side, for converting output current to sinewave current; and an AC power source interconnected to the power system.
However, energy loss occurs due to return of current between the smoothing capacitor and the interconnection reactor, and thus the power conversion efficiency reduces. Considering this, it is known that a separation circuit for separating the smoothing capacitor and the interconnection reactor from each other during flywheel is provided to improve the power conversion efficiency of the inverter circuit (for example, U.S. Pat. No. 7,046,534).
FIG. 13 shows an example of an inverter circuit (HERIC: registered trademark) having the conventional separation circuit. A bridge circuit 104 includes four switching elements Q101 to Q104 and four flywheel diodes D101 to D104 respectively connected in reversely parallel to the switching elements Q101 to Q104. In a separation circuit 106, two reversely parallel connection paths are provided between two AC output lines 11 and 12 of the bridge circuit 104, and on the respective reversely parallel connection paths, a separation switching element Q105 and a separation diode D105 are connected in series, and a separation switching element Q106 and a separation diode D106 are connected in series. The separation diodes D105 and D106 allow current flow in directions opposite to each other. Plus flywheel current flows through the separation switching element Q105 and the separation diode D105, and minus flywheel current flows through the separation switching element Q106 and the separation diode D106. During a period in which the flywheel currents flow, the smoothing capacitor 3 and the interconnection reactor 5 are separated from each other, and energy loss is reduced.